Reduction of permeability in subterranean formations is desirable in a number of fields.
In the oil industry, in the course of some secondary oil recovery operations, water is injected through an injection well to sweep or drive oil toward an adjacent production well. A serious problem that can arise in such an operation is that the water preferentially moves through permeable strata in the formation and bypasses oil contained in less permeable strata. This narrowly focused water movement is commonly referred to as "fingering". As a result of fingering, the sweep efficiency of many water-swept operations fall far short of what is sought.
Another water movement problem associated with oil recovery operations is referred to as "coning". When an oil well is being produced, water present in a stratum underlying the oil zone can "cone" upwardly and enter the well bore. As the difference in viscosity between the oil and water is usually significant the water tends to move more easily through the rock or sand matrix adjacent the well bore. As a result, this flow of water excludes the oil from the well bore.
Because of these problems, there is an ongoing search in the oil industry for an effective means for preventing the movement of water or displacement fluids through permeable zones or strata associated with an oil reservoir.
In addition, reducing formation permeability is also desirable in other areas. These include to prevent seepage of salt water or waste to water supplies, or of water from water-retaining structures.
Methods have been developed, particularly in the oil industry, to reduce formation permeability. However, they entail a number of problems.
One method in the prior art involved the injection of surfactant-containing foams into the target formation. Such foams are normally formed using an inert gas, a surfactant, and a liquid. They may be injected as a preformed foam, or by sequential injections of surfactant solution and gas. The surfactant causes the surface tension of the foam bubbles to drop, so that the foam can easily penetrate the permeable zones in the formation. The problem with the use of such foams is that the foam remains unstable and is therefore mobile or readily displaceable. Thus it may be displaced by water or displacement fluids used in attempting to produce the well.
Another method is disclosed in U.S. Pat. No. 4,800,959 entitled "Microbial Process for Selectively Plugging a Subterranean Formation". This patent noted that laboratory studies have shown that bacterial expolysaccharides that coalesce to form a confluent biofilm can be used to effectively seal a simulated reservoir matrix or core formed of fused glass beads (as disclosed in "Bacterial Fouling in a Model Core System", J. C. Shaw et al, (1985) Applied and Environmental Microbiology, p. 693-701). However, if vegetative cells are used, "skin plugging" occurs--a build-up of thick biofilm at the injection point. U.S. Pat. No. 4,558,739 issued to Mclnerney et al sought to eliminate this problem by injection of bacterial spores, which are metabolically inert and non-adhesive in nature. However, problems remained--of size constraints, as the spores are still of 1 .mu.m in diameter; only a few types of bacteria produce spores; and specific nutrients are necessary to return the spores to the vegetative state. Those problems were attacked in U.S. Pat. No. 4,800,959, by use of ultramicrobacteria, or UMB.
UMB are produced by certain bacterial strains in a low-nutrient environment. Under such a starvation regime, the cells undergo significant reductions in cell size and morphological transformations during progressive cell divisions, to form the reduced-size cells known as UMB. The diameter of UMB range from about 0.2 .mu.m to about 0.4 .mu.m. In the absence of nutrient, UMB do not adhere readily to a sand matrix such as found in a reservoir.
U.S. Pat. No. 4,800,959 disclosed injecting UMB into the formation, followed by a specific nutrient controlled solution to resuscitate the UMB to the vegetative state. The revived cells then produce biofilm to plug the formation. Preferably the UMB were formed by isolating the bacterial class indigenous to oil reservoir waters, such as Pseudomonas putida or a Klebsiella species and subjecting them to a starvation regime.
One of the problems with using UMB as disclosed in this patent, is that the plugging of the formation depends on the continued existence of the biofilm. Another problem is that there is a significant time lag before plugging takes place, as the plug does not form until the UMB resuscitate and the cells produce exopolymer.